Heat-treatable aluminum alloys belong to a large class of age-hardenable materials comprising base metals (Al, Fe, Ti, Mg, Cu, Ni, Mo, W and other) and alloying elements having a strong dependence upon solubility related to temperature. At high temperatures, these elements can be fully dissolved, then fixed into a solid solution by quenching, and, finally, precipitated into a matrix of the base metal during aging at specific temperature and time. Aging forms very fine precipitates which provide a significant strengthening effect. For heat treatable aluminum alloys, such processing is the typical T6 temper route that is usually used following forming or machining operations. However, because of high temperature solution treatment, materials and components after T6 temper have coarse grain structures. To prevent grain growth during solution treatment and exposures to increased temperatures, most precipitation hardening alloys comprise insoluble elements that form particles and dispersions of second phases. These brittle intermetallic phases, typical of a size more than 5 microns, are stress concentrators and origins of micro-cracks under monotonic and cyclic loading resulting in insufficient ductility, toughness, fatigue and stress corrosion.
It is known in the art that improvement in the properties of precipitation hardening alloys may be attained by thermo-mechanical processing (TMP) using plastic deformation after solution treatment. Depending on the temperature of deformation, there is cold and hot TMP. For cold thermo-mechanical processing (CTMP), deformation is performed prior to aging, during aging and after aging at temperatures below or equal to the aging temperature. Different variants of cold TMP were described in U.S. Pat. Nos. 3,706,606; 4,596,609, U.S. Patent Application No. 20100243113, International Application WO/2009/132436, and others. In comparison with T6 temper, cold TMP hardens the matrix, refines and more uniformly distributes precipitates and increases the material strength. An especially strong hardening effect of cold TMP is observed when intensive deformation is performed by Equal Channel Angular Extrusion as it has been disclosed in U.S. Patent Application No. 20070084527. However, CTMP: (i) develops substructures within grains but does not refine coarse grains induced during solution treatment; (ii) requires high stresses and loads; (iii) may result in cracks because of insufficient material ductility; and (iv) cannot be applied to complicated components and for operations of net shape forming.
Hot thermo-mechanical processing (HTMP) is usually performed by forging, rolling or extrusion at high temperatures followed immediate quenching and aging (FIG. 1). The most known version of HTMP is intermediate thermo-mechanical processing (ITMP) often designated as T5 temper. With proper strain rate and quenching time after deformation, ITMP produces dynamically recrystallized fine grain structures which improve the material toughness and fatigue. It also resolves other issues of CTMP. However, forging temperatures and heating time during ITMP are not sufficient to transfer all soluble elements into the solid solution. Part of the soluble elements form large precipitates which do not contribute to the hardening effect, and the material strength after hot TMP is noticeably lower than that for T6 condition. Therefore, ITMP has found restricted industrial applications and its potential for HTMP remains unrealized. An ordinary practice is to use T6 heat treatment after hot forming and machining operations as shown in FIG. 2, if the primary interest is the material strength.
The present invention combines advantages of cold and hot TMP and eliminates the mentioned shortcomings. From foregoing explanations, it is clear that such processing technique would be very desirable in the art.